Contact sensors, particularly touchscreens are becoming a popular input device for computers. It takes advantage of simple intuitive human response. Applications include information kiosks, point-of-sale equipment, vehicular information and control, and control machinery of all sort, as well as the obvious ones of interacting with computers in word-processing, spread-sheet, and a host of other applications. Many prior art inventions related to this area are disclosed, and several touchscreens are commercially available to date.
Touchscreens used in conjunction with a computer controlled display for interactive information exchange between man and computer functions as position encoder. In order to use the input and display interactively, there must be a direct mapping of positions from one surface to the other. For human user, this relationship is simplified considerably if the two surfaces are coincident. This assumes a one to one mapping scale. Touchscreens were invented to achieve one to one mapping scale by detecting the x and y coordinates of the point on the display touched by a probe, finger or other stylus. There are many kinds of touchcreens utilizing various sensing mechanism such as resistive, capacitive, acoustics and optics. In according with present invention, I will only discuss the resistive touchscreens.
Resistive touchscreen is a transparent type of telescriber shown in U.S. Pat. No. 2,900,446, elecrographic sensor described in U.S. Pat. No. 3,798,370 or contact sensor shown in U.S. Pat. No. 3,911,215, mounted on the top of the display. All these sensors are special devices that generates the electrical signals which are proportional to some physical point in a planar or non-planar coordinates system. Numerous devices have been devised that are acclaimed to solve individual of these and similar applications. One of the earlier of these devices is shown and described in U.S. Pat. No. 2,269,599 to H. C. Moodey. Another of the typical prior art single layer x-y position sensitive devices is that described in a booklet entitled "Information Display Concepts," distributed by Tektronics, Inc. (1968), and referred to as an "x-y tablet." Still another patent as mentioned early is the device described in U.S. Pat. No. 2,900,446 to D. J. McLaughlen, et al., In all of these devices, continuous electrodes are placed along each edge of a resistive sheet and various means are described for applying voltages between the electrodes to obtain the necessary orthogonal electrical fields. These same electrodes, however, cause severe distortion to the electrical fields during the time interval when they are not connected to the voltage supply. This restricts the use to only a small central region of the resistive sheet for accurate determinations of point coordinates.
The device described in U.S. Pat. No. 3,449,516 to S. H. Cameron, et al., is designed to reduce the field distortion caused by the continuous electrodes. Switching devices are used with each of several discontinuous electrodes to effect application of electric potentials to a resistive sheet. Each electrode is completely isolated from others when no voltage is being applied. Another proposed solution to the problem of distortion is the device described in U.S. Pat. No. 3,591,718 to Shintaro Asano. In his device, the resistive sheet is framed with strips of a material having a lower resistivity than the sheet. The potentials for producing the electrical fields are applied to electrodes at the corners of the frame. The potential at any position along the edge, however, is affected by the quality of the contact between the strips and the sheet and the uniformity of the resistivity of the strips. Still, in order to achieve high linearity throughout a layer area of the device, many more special systems of electrodes have been devised to increase the region of linearity of the instrument. For example, in U.S. Pat. No. 3,798,370 issued to G. S. Hurst on Mar. 19, 1974. Electrodes for the application of the voltage to the sheet arranged in a curve or bow whereby the voltage drops in the resistive element along the edges of the device are at lease partially compensated. In a like manner, special electrode configurations are shown and described in U.S. Pat. No. 4,079,194 on Mar., 14, 1978, in U.S. Pat. No. 4,178,481, on Dec. 11, 1979 and in U.S. Pat. No. 4,214,122, on Jul. 22, 1980 all to V. Klay. In all these patents, special electrodes configurations are used to reduce the bow to increase the sufficiently linear area of given sized sensor. Another patent that describes the special electrode configurations is U.S. Pat. No. 4,293,734, issued to W. Pepper, Jr., on Oct. 6, 1981. This is one of a series of patents issued to Pepper. These electrodes occupy a considerable space along the edge of the sensor. Also, in Pepper the network disclosed combine both the peripheral resistance network and the electrodes for introducing the potential into the resistive layer whereby a change in one effects the other and thus does not provide individual adjustment.
Other patents in trying to improve the linearity of resistive sheet by special electrode configurations are set forth below:
U.S. Pat. No. 4,661,655 PA1 U.S. Pat. No. 4,731,508 PA1 U.S. Pat. No. 4,777,328 PA1 U.S. Pat. No. 4,797,514 PA1 U.S. Pat. No. 4,822,957
In addition to these single layer devices, there are known to be many multilayer resistive position touch sensors for generally accomplishing the desired results. When using multilayer system, the contact sensor comprises at lease first sheet of flexible material and second sheet. The first sheet is capable of being energized to establish an electrical potential thereon, or the first flexible sheet functions as a conductive probe. The second sheet can be energized to establish an electrical potential in juxtaposition with the first sheet. To keep the sheet apart, a plurality of substantially uniform discrete insulating spacers are used. For a touchscreen mounting on a display, the flexible sheet and the insulating spacers severely reduce the brightness of display. Also the flexible sheet made from plastic materials can be easily damaged. Further the flexible sheet and the insulating spacers significantly increases the cost of sensor. All these drawbacks make a multilayer touchscreen undesirable. In according with present invention, a preferred embodiment of single layer touchscreen is disclosed.
The multilayer touchscreen does have one special feature over single-layer touchscreen, that is the multilayer sensor is able to sense any of styluses including fingertip. Although touching by a finger is obviously convenient, it also has many unseen disadvantages. For example: 1) the fingertip is too large to accurately touch a small target on the display; 2) touching by a finger usually leaves dirt, water or fingerprint on the screen, this becomes a severe problem in harsh environment; 3) when touching a touchscreen mounted on a movable display such as the one in a lap-top computer, one often is required to hold the display by one hand and use another hand to touch because it is difficult to control the light touch pressure by a finger whereas a stylus pen can just lay on the screen to perform the contact with screen. In any cases, the special feature of sensing the touch by a finger keeps the multilayer touchscreen in today's market and probably in some special applications in the future too. Therefore, in according with present invention, a secondary embodiment of multilayer touchscreen is disclosed.
In all the resistive touch sensors, single layer or multilayer, the potential field is alternately switched between vertical equipotentials and horizontal equipotentials in synchronism with the detection circuit to provide an X analog signal output and a Y analog output representative of the horizontal and vertical coordinates, respectively of the touch tip above the sensor. This switching mechanism not only delays the touch response but also increases the system complexity as well as the cost. The delay in touch response is a major drawback in hand writing application.